Question: Divide the following complex numbers. $ \dfrac{-15+25i}{-4+i}$
We can divide complex numbers by multiplying both numerator and denominator by the denominator's complex conjugate , which is ${-4-i}$ $ \dfrac{-15+25i}{-4+i} = \dfrac{-15+25i}{-4+i} \cdot \dfrac{{-4-i}}{{-4-i}} $ We can simplify the denominator using the fact $(a + b) \cdot (a - b) = a^2 - b^2$ $ \dfrac{(-15+25i) \cdot (-4-i)} {(-4+i) \cdot (-4-i)} = \dfrac{(-15+25i) \cdot (-4-i)} {(-4)^2 - (1i)^2} $ Evaluate the squares in the denominator and subtract them. $ \dfrac{(-15+25i) \cdot (-4-i)} {(-4)^2 - (1i)^2} = $ $ \dfrac{(-15+25i) \cdot (-4-i)} {16 + 1} = $ $ \dfrac{(-15+25i) \cdot (-4-i)} {17} $ Note that the denominator now doesn't contain any imaginary unit multiples, so it is a real number, simplifying the problem to complex number multiplication. Now, we can multiply out the two factors in the numerator. $ \dfrac{({-15+25i}) \cdot ({-4-i})} {17} = $ $ \dfrac{{-15} \cdot {(-4)} + {25} \cdot {(-4) i} + {-15} \cdot {-1 i} + {25} \cdot {-1 i^2}} {17} $ Evaluate each product of two numbers. $ \dfrac{60 - 100i + 15i - 25 i^2} {17} $ Finally, simplify the fraction. $ \dfrac{60 - 100i + 15i + 25} {17} = \dfrac{85 - 85i} {17} = 5-5i $